Change the buffer is always a good idea. You might have traces from old DNA or proteins from previous runs. Also be sure that the gel is not loosing adhesion with the tank. (on the areas were the gell is not attached to the tanks if there is space the current will run differently and will drag the protein/DNA sideways or in a V shape)

Dear, low quality pic but 1. change your electrophoresis tanks, 2. Use of lower voltage for high MW protein. i have same problem and use another tank then solved.

Make fresh buffers, fresh solutions for gel casting, run elfo at lower voltage

You can change the buffer and use fresh buffer for running the gel and be sure your samples doesn't have a DNA as well, might be these smear is because of DNA and you can use DNase.

Best

Change the buffer is always a good idea. You might have traces from old DNA or proteins from previous runs. Also be sure that the gel is not loosing adhesion with the tank. (on the areas were the gell is not attached to the tanks if there is space the current will run differently and will drag the protein/DNA sideways or in a V shape)

Simple trick, but 1 minute to try it without any risk (I do it very often in case of problematic samples only due to my laziness;) You load your gel at 80-100V at the beggining, wait until the blue front will go 1mm (no more) in the stacking gel. You open the apparatus, wash the gel slots very strongly to make sure the foam appears, anyway at the beginning watch that even the glycerol is not such an easy to be removed. I usually do it with 10ml pipette with pipetor or 1ml Gilson. The idea of doing it is you have some non-protein contaminants go slowly, and you wash them of before they go into the gel causing bad separation. This is in agreement of your result, where small actin migrating fast is not affected, whereas your big one is said;(

Of course the problem is with your tank ! the voltage is not the same all over the surface !